Redesign: add more-content-above-below indicators in scrollable areas

[bwindels]

Like so: http://dabblet.com/gist/2462915

[bwindels]

Input from Nad: try with a background to transparent gradient

[bwindels]

Hmm, just had a quick go at this, and doing background to transparent might be a bit more involved, because the gradient is drawn behind the text, so anything of the same color as the background is not visible.

To have a gradient drawn on top of the children of a scroll area, we'd have to position them absolutely and set a z-index and pointer-events: none, but only show them when an element is overflowing. Firefox supports overflow events which we already use as a fallback for the native scrollbars, but this won't work elsewhere ...

[bwindels]

Actually, we should rely on scroll event, and checking overflow manually after mounting the list to show/hide the indicators.

[bwindels]

PR works well, just need some finish to also update when content is added/removed from a list, or the list is resized.